The present observation indicates that, besides the nitrogenous compounds, other biologically active cell constituents like vitamin B₁₂ and auxins may also appreciably contribute to the fertilizing action of the nitrogen-fixing blue-green algae. Of the various intracellular amino acids of Cylindrospermum muscicola, cystine, tyrosine and phenylalanine seem to be available to the rice plants. The alga can synthesize vitamin B₁₂ active compounds, the yield of which was about 1.2 to 1.5μg/g dry weight of the alga. The synthesis of these compounds was promoted by the addition of cobalt salt. The alga also contains interconvertible auxin-like substances which stimulate the root growth of rice seedlings.